Plays about the legacy of the 1960s are becoming increasingly common. After Mike Bartlett's Love, Love, Love and Alexi Kaye Campbell's Apologia, we now have this debut from Stephen Beresford. As an actor himself, he knows how to write whacking good parts and has all the benefits of a meticulous National production, but he rarely makes you feel the family he portrays can provide a metaphor for a generation.

The setting, vividly created by Vicki Mortimer, is a dilapidated art deco house on Devon's south coast. Its chief occupant is Judy, an unreconstructed old hippy who has just come through a minor cancer operation which reunites her with her family. Her waspish daughter, Libby, has recently been dumped, has her own caustic daughter in tow and is anxious about her inheritance. Judy's gay son, Nick, a former drug addict living from hand to mouth, seems even more desperate. Throw in a dodgy local doctor and a troubled, pool-cleaning teenager, and you have a house filled with emotional as well as physical chaos.

Beresford never seems wholly sure what kind of play he wants to write. Partly we get yet another English variation on The Cherry Orchard, with a feckless family seeing their rambling property grabbed from under their noses. But Beresford also sets out to show the damage inflicted by 1960s revolutionaries on their immediate successors. He certainly shows a gift for vivid phrases, such as Nick's accusation to his mother that, "While you were wanking into a chrysanthemum, Margaret Thatcher was making her entrance." But one woman, who flogged radical newspapers and hauled her children around India, is hardly an adequate symbol for an era that brought about a genuine social revolution; and, having put his heroine in the dock, Beresford then seems to fall half in love with her.

As always, however, director Howard Davies gets luminous performances from his cast. Julie Walters, sensuously fondling her breasts as she urges the young pool-cleaner to "let the body take over", conveys the humour as well as the madness of the heroine. But it is Helen McCrory who really carries the evening as the daughter who views her mother with a mix of vituperative anger and exasperated affection. Rory Kinnear also excellently conveys a sense of wasted intelligence as the ex-junkie son, and there is exemplary support from Matthew Marsh as the fake-revolutionary doctor. There's enough here to make you keen to see what Beresford does next, but I still long for a play that does justice to a decade that changed Britain irrevocably for the better.